Effect of hydration on the single-crystal elasticity of Fe-bearing wadsleyite to 12 GPa

The single-crystal elastic properties of Fe-bearing wadsleyite with 1.93 wt% H2O (Mg1.634Fe0.202H0.305SiO4) have been determined by Brillouin scattering. At ambient conditions, the aggregate bulk and shear moduli (KS0, G0) of this wadsleyite are 156.2(5) and 98.0(3) GPa, respectively. Compared to the corresponding anhydrous wadsleyite, 1.93 wt% H2O lowers KS0 and G0 by 8.1% and 9.3%, respectively. High-pressure measurements up to 12 GPa show that the pressure derivative of the bulk modulus, K′S0 = 4.8(1), is similar to that of the anhydrous Fe-wadsleyite with reported values of 4.6–4.74, but the addition of H2O increases the pressure derivative of the shear modulus, G0′ from 1.5(1) to 1.9(1). This contrasts with the G0′ of Fe-free wadsleyite, which is the same within uncertainty for the hydrous and anhydrous phases. As a result, both the compressionaland shear-wave velocities (vP, vS) of hydrous Fe-bearing wadsleyite are about 200(±24) m/s slower than anhydrous Fe-bearing wadsleyite at transition zone pressures.